Detector units for use in detecting unauthorised intrusion into homes, offices, factories and other buildings are well known. These units normally comprise a detector such as passive infrared detectors, door and window condition indicators, window trembler switches, smoke or fire detectors, window trip wires and the like and a signalling means that emits a signal (hereinafter called "a detector signal") when the detector is actuated by e.g. detecting an untoward occurrence. The detectors are connected to a main station control unit which when it receives a detection signal actuates an alarm or similar device that indicates an untoward occurrence (hereinafter called "a fault") which has caused the detector to emit its detector signal.
Many if not most of these detectors are normally connected by wire connectors to the main station control unit. The detector also receives the power for its operation by wire connectors from the main control units. Thus the installation of the detector unit is a skilled operation requiring skilled workers to prepare the installation. Further, once the detector unit is installed, it is difficult to reposition without causing damage to the wall or other part on which it is mounted and along which the wires run. It has been proposed that the signalling means should be a battery driven radio frequency transmitter which avoids many of the problems mentioned above. However such detector units have not been widely accepted due inter alia to the fact that when the battery loses power, the zones covered by the detector units are often unsurveyed.
In practice with protection devices of acceptably moderate cost, the main control unit will have only a limited number of input lines or radio channels to it so that a number of detector units will be connected to the main control unit through common lines. These detector units will normally be arranged to protect a single zone--for example one floor in an apartment building, the sleeping quarters in a house, the store room in a factory. The main control unit will thus be able to indicate that a detector signal has been given from a zone but will not identify which detector unit in the zone has emitted the detector signal. Consequently the user will only be able to ascertain from the main control unit the zone where the untoward occurrence took place without knowing in detail where this has occurred.
An object of the invention is to provide a detection unit which is capable of overcoming one or more of the foregoing disadvantages.